A dip pen reservoir is designed to slip on the nib that are using, the purpose of a reservoir is to hold an excess amount of ink. There are 2 types of reservoir nibs, 1. Nibs that come with the reservoir pre-attached such as Leonardt Tape nibs or Brause Bandzug nibs and 2. Those that the end user can if they wish attach a reservoir too, such as the William Mitchell Round Hand.
Some nibs such as the William Mitchell Round Hand do hold ink well in comparison to the William Mitchell Italic nibs, therefore it is dependent on the nib being used. Another factor to consider is the wider the nib, the more ink will be transferred to the page, compared to a narrow nib; therefore a reservoir would be of more help on a broader nib.
The fitment of a reservoir is important, the reservoir should not be too loose that it simply falls off and conversely should not be too tight that it distorts the shape of the nib that you are using. The distortion would most likely result in the tines at the end of the nib splitting, resulting in 2 separate unwanted lines.
Reservoirs are typically made of brass, and are therefore malleable; the two main types are the William Mitchell and Manuscript reservoirs both of which are made of brass. On the actual reservoir you will find a lug, at each side; they can be bent in order that the reservoir attaches itself to the nib.
It it is important that the tip of the reservoir, touches the underside of the nib; in order that that ink can be fed to the nib. The reservoir should be about 3mm or 1/8″ from the actual tip of the nib. It is a matter of choice whether you dip or brush ink, onto the underside of the nib with the reservoir attached. It is quicker to dip the pen into ink, however there may be a risk of excess unwanted ink on the page, therefore a small piece of tissue / cloth to wipe unwanted ink from the nib can be useful.
IAMPETH has more an article on achieving hairlines with broad edged nibs, there is a short section on positioning of reservoirs; it is quite helpful. If you would like to read more about this; please click here. Thanks for reading.